The present disclosure relates to a coating for an article or a part, which coating provides improved wear performance.
Chromium plating has been used very successfully for over 50 years in the prevention of wear on a variety of components. One example involves hydraulic actuators which rely on a hard coating to prevent scoring and general wear of actuator piston shafts and actuator bores. Any damage to these surfaces can result in excessive seal leakage and premature failure.
High Velocity Oxy-Fuel (HVOF) tungsten carbide thermal spray processes have been used with great success as chromium plate replacements. However, thermal spray processes are limited primarily to line-of-sight applications and can cost up to three times that of chromium plate. The highest costs are incurred in housing bore applications where the bore length divided by diameter is greater than one.
Increasingly tighter restrictions on many known environmentally hazardous materials or processes have forced manufacturers to require only environmentally friendly processes be used in the manufacture of their own equipment and equipment which they purchase. Among these are processes which incorporate hexavalent chromium or hex-chrome.
Hex-chrome is the primary functional constituent found in chromium plating baths. These baths create a mist during the plating process containing hex-chrome, which must be captured and processed through a complex and costly waste treatment system prior to disposal. Additionally, parts removed from the plating baths must be water rinsed. The rinse water must be treated similarly to the captured mist as hazardous waste before the water can be appropriately discharged. Also, making up chromium plating baths exposes workers to the hazards of handling hexavalent chromium containing compounds.
Composite electro-plated nickel or cobalt platings containing hard particles such as silicon carbide or chromium carbide have had limited success in replacing chromium plate. While the hard carbide particles in these coatings prevent excessive abrasion, the soft nickel or cobalt plating matrix which holds the particles in place can be easily scratched causing an imperfect surface which could facilitate seal leakage. In addition, as the soft matrix wears, the carbide particles can become loose. Loss of a carbide particle leaves a void in the surface contributing toward seal leakage, and allows the hard carbide to act as a third body abrasive particle.
Hard platings, like electroless nickel-boron or electroless nickel-phosphorous, without hard particles added, have also been used with limited success. These finishes have traditionally been limited to a very thin buildup (less than 0.003 inches thick). Such a buildup cannot be machined significantly after deposition, limiting its use in dimensional restoration on worn surfaces. Even on new hardware tighter manufacturing tolerances are required in order to prevent machining through the plating. Without the addition of hard particles, these coatings still tend to wear more significantly than chrome plate or HVOF tungsten carbide. In addition, electroless nickel-phosphorous has been known to experience adhesive wear like galling, and the electroless nickel-boron tends to fail by brittle fracture of the columnar structure resulting in pull out of the coating.
Due to recent environmental regulations, there is a need to replace conventional chromium electroplate for all applications involving a wear resistant coating.